a level biology topic 2

• Eukaryotic cells


  • Cytoplasm containing membrane-bound organelles
  • DNA enclosed in a nucleus
• Cell-surface membrane


  • Selectively permeable, enables control of passage of substances in/out of cell
  • Molecules/receptors/antigens on surface allow cell recognition/signalling

  
• Nucleus


  • Holds/stores genetic information which codes for polypeptides (proteins)
  • Site of DNA replication
  • Site of transcription (part of protein synthesis), producing mRNA
  • Nucleolus makes ribosomes/rRNA

  
• Ribosome


  • Made of ribosomal RNA and protein (two subunits)
  • Not a membrane-bound organelle
  • Site of protein synthesis (translation)

  
• Rough endoplasmic reticulum (rER)

  • Ribosomes on surface synthesise proteins
  • Proteins processed/folded/transported inside rER
  • Proteins packaged into vesicles for transport eg. to Golgi apparatus

  
• Smooth endoplasmic reticulum (sER)

  • Synthesises and processes lipids
  • Eg. cholesterol and steroid hormones
• Golgi apparatus


  • Modifies protein, eg. adds carbohydrates to produce glycoproteins
  • Modifies lipids, eg. adds carbohydrates to make glycolipids
  • Packages proteins/lipids into Golgi vesicles
  • Produces lysosomes (a type of Golgi vesicle)
• Golgi vesicles


  • Transports proteins/lipids to their required destination
  • Eg. moves to and fuses with cell-surface membrane

  
• Lysosomes


  • Release hydrolytic enzymes (lysozymes)
  • To break down/hydrolyse pathogens or worn-out cell components
• Mitochondria


  • Site of aerobic respiration
  • To produce ATP for energy release
  • Eg. for protein synthesis/vesicle movement/active transport
• Chloroplasts


  • Absorbs light energy for photosynthesis
  • To produce organic substances eg. carbohydrates/lipids
• Cell wall


  • Composed mainly of cellulose (a polysaccharide) in plants/algae
  • Composed of chitin (a nitrogen-containing polysaccharide) in fungi
  • Provides mechanical strength to cell
  • Prevents cell changing shape or bursting under pressure due to osmosis
• Cell vacuole


  • Maintains turgor pressure in cell (stopping plant wilting)
  • Contains cell sap → stores sugars, amino acids, pigments and any waste chemicals
• Organisation of eukaryotic cells in complex multicellular organisms

  1. Tissue

  2. Organ

  3. Organ system
• Eukaryotic cells become specialised for specific functions in complex multicellular organisms
• Prokaryotic cells


  • Cytoplasm lacking membrane-bound organelles
  • Genetic material not enclosed in a nucleus
• Examples of prokaryotic organisms

  • Bacteria
  • Archaea
• Viruses


  • Acellular - not made of cells, no cell membrane/cytoplasm/organelles
  • Non-living - have no metabolism, cannot independently move/respire/replicate/excrete
• Structure of a virus particle

  • Nucleic acids surrounded by a capsid (protein coat)
  • Attachment proteins allow attachment to specific host cells
  • No cytoplasm, ribosomes, cell wall, cell-surface membrane etc.
  • Some also surrounded by a lipid envelope eg. HIV

  
• Magnification


  Number of times greater image is than size of the real (actual) object
• Resolution


  Minimum distance apart 2 objects can be to be distinguished as separate objects
• Calculating magnification, real size & image size

  1. Note formula/rearrange if necessary

  2. Convert units if necessary

  3. Calculate answer and check units required or if standard form etc. is required
• Unit conversions


  • Centimetre (cm) = 0.01 m
  • Millimetre (mm) = 0.001 m
  • Micrometre (µm) = 0.000001 m
  • Nanometre (nm) = 0.000000001 m
• Measuring size of object with optical microscope

  1. Line up (scale of) eyepiece graticule with (scale of) stage micrometer

  2. Calibrate eyepiece graticule - use stage micrometer to calculate size of divisions on eyepiece graticule

  3. Take micrometer away and use graticule to measure how many divisions make up the object

  4. Calculate size of object by multiplying number of divisions by size of division

  5. Recalibrate eyepiece graticule at different magnifications
• Cell fractionation and ultracentrifugation

  1. Homogenise tissue/use a blender

  2. Place in a cold, isotonic, buffered solution

  3. Filter homogenate

  4. Ultracentrifugation - separates organelles in order of density/mass
• Eyepiece graticule division


  100 µm/4 = 25 µm
• Describe and explain the principles of cell fractionation and ultracentrifugation as used to separate cell components
• Cell fractionation


  1. Homogenise tissue / use a blender

  2. Place in a cold, isotonic, buffered solution

  3. Filter homogenate

  4. Ultracentrifugation - separates organelles in order of density / mass
• Homogenisation


  • Disrupts cell membrane, breaking open cells and releasing contents / organelles
• Cold, isotonic, buffered solution


  • Cold to reduce enzyme activity → so organelles not broken down / damaged
  • Isotonic so water doesn't move in or out of organelles by osmosis → so they don't burst
  • Buffered to keep pH constant → so enzymes don't denature
• Filtration


  • Remove large, unwanted debris eg. whole cells, connective tissue
• Ultracentrifugation


  1. Centrifuge homogenate in a tube at a high speed

  2. Remove pellet of heaviest organelle and respin supernatant at a higher speed

  3. Repeat at increasing speeds until separated out, each time pellet made of lighter organelles (nuclei → chloroplasts / mitochondria → lysosomes → ER → ribosomes)
• Common mistakes in electron microscope descriptions
• Describe the stages of the cell cycle in eukaryotic cells
• Cell cycle in eukaryotic cells

  1. Interphase (S phase, G1/G2)

  2. Mitosis

  3. Cytokinesis
• Interphase


  • DNA replicates semi-conservatively, leading to 2 chromatids (identical copies) joined at a centromere
  • Number of organelles & volume of cytoplasm increases, protein synthesis
• Mitosis


  • Nucleus divides to produce 2 nuclei with identical copies of DNA produced by parent cell

  
• Cytokinesis


  • Cytoplasm and cell membrane (normally) divide to form 2 new genetically identical daughter cells
• Stages of mitosis


  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase
• Prophase


  • Chromosomes condense, becoming shorter / thicker (so visible)
  • Appear as 2 sister chromatids joined by a centromere
  • Nuclear envelope breaks down
  • Centrioles move to opposite poles forming spindle network